As produced, carbon blacks are powdery materials with bulk densities ranging from about 0.02 to 0.1 g/cc and are termed fluffy blacks. Because of their low densities and large surface areas, the fluffy products are cohesive, have very poor conveying properties and are quite dusty. They are, however, dispersible. Because of their poor bulk handling properties, advantage of their excellent dispersibilities cannot be taken in many applications. For example, it is difficult to feed fluffy blacks in a controlled manner to standard dispersing devices, such as Banbury mixers, twin screw extruders or the like.
To improve their handling properties, the fluffy products are densified. For a given grade of black, handling properties tend to improve with increasing degrees of densification. Dispersibility, on the other hand, is progressively degraded as the extent of densification is increased. Thus there is a tradeoff between improvement in bulk handling and degradation in dispersibility. For this reason, the extent and means employed to densify the fluffy products depend on their intended uses.
The industry, in general, uses three basic methods to attain densification. These, in order of providing increased levels of densification, are: agitation or vacuum treatment of the fluffy product, dry pelletization and wet pelletization. Since the performance of carbon black in many applications depends on the degree of dispersion attained, the acceptable extent of densification achieved depends on the user's dispersion equipment and, especially, on the shearing stresses generated.
The process of agitation or vacuum treatment yields a powder which is difficult to bulk handle and is commercially supplied in a bagged form. Nevertheless, because this form of the product is much more dispersible than its more dense counterparts, it is used in applications where easy dispersion is mandatory.
Dry pelletization gives pellets which are relatively weak and have poor attrition resistances. As a consequence, conveying can cause pellet breakdown which leads to a degradation in their bulk handling properties.
Wet process pellets are formed in pin pelletizers using water as a cohesive fluid. The resultant pellets, after drying, are relatively dense, hard and attrition resistant. Such products have relatively good bulk handling properties but are more difficult to disperse than their dry process counterparts.
Dry pelletization is conducted in rotating drums. Industrial drums have diameters of 1.83 to 3.04 m (6 to 10 feet) and lengths of 6.1 to 12.2 m (20 to 40 feet) and are rotated at 5 to 20 RPM (revolutions per minute). Fluffy black together with seed pellets are fed continuously to one end of the drum. Tumbling of the fluffy black/seed pellet mixture in the rotating drum results in the formation of product pellets which exit the other end of the drum. The seed pellets employed commercially consist of part of the product pellets which are recycled to the feed end of the drum.
Pellet formation in the dry drum process is greatly facilitated by the use of seed pellets. Generally, the fluffy black to seed pellet weight ratio is in the region of 1:1 but can vary from 0.2:1 up to 5:1. The seed pellets serve as the nuclei for pellet formation and growth. The quality of the seed pellets, in terms of density, size and size distribution, and the amount employed has a profound effect on product quality. Since part of the product pellets are employed as seed pellets, the qualities of the seed and product pellets are coupled. Hence, formation of poor quality pellets leads to formation of poor quality seed pellets which, in turn, results in a further reduction in product quality. Thus there is a strong feedback loop between seed and product quality, the decoupling of which would be highly advantageous. This decoupling can be achieved by using an alternate source of seed pellets so that the extent of product recycle can be either reduced or, more preferably, eliminated.
Voyutsky et al. claimed that the seed material used in the dry drum pelletizing of carbon black could consist of either densified carbon black of small diameter or "any small foreign bodies such as plant seeds, sugar crystals, etc." Voyutsky et al. and Voyutsky and Rubina showed that the black was deposited onto the seed material. These disclosures are found in Voyutsky, S. S., A. D. Zaronchkovsky and S. I. Rubina, "Causes of the Granulation of Powders," Colloid J. (USSR), 14, 28 (1952) and in Voyutsky, S. S. and S. I. Rubina, "Pelletization of Powders by the Rolling Method, Light Industry (USSR), 12, 36 (1952). These workers varied the sizes of their seed pellets from about 0.8 to more than 2.5 mm and found, when using a 1:1 black to seed ratio, that the fractional increase in the diameters of the resultant pellets increased with seed diameter. However, as seed diameter increased, the size distribution of the resultant pellets showed a progressively increasing degree of tailing at the fine end of the distribution. This tailing was attributed to crushing of the carbon shells of some of the larger pellets. These workers also claimed that pellet formation, in the presence of seed, was more rapid and yielded larger pellets at elevated pelletizing temperatures. Temperatures from 20.degree. up to 95.degree. C. were investigated.
Ross and Davies studied the dry drum pelletization of a furnace black in a 0.094 m (3.72-inch) diameter drum. This disclosure is found in Ross, T. K. and T. Davies, "The Granulation of Carbon Black," Trans. Instn. Chem. Engrs., 39, 28 (1961). They used 2.7 mm glass spheres as seed and found that the seed became coated with a relatively dense layer of carbon black.
In their studies of the dry pelletization of zinc oxide, Meissner H. P., A. S. Michaels and R. Kaiser in "Rate Of Pelletization Of Zinc Oxide Powders," Ind. Eng. Chem. Process Design and Development, 5, 10, (1966) and in "Spontaneous Pelletization In Fine Powders," Ind. Eng. Chem Process Design and Development, 3, 197 (1964), claimed that an uncompacted mass of any powder having particles less than one micron in size can be converted into closely sized, dense, free-flowing pellets by tumbling in a drum. They suggested that the attractive forces holding the particles together in an agglomerate or a pellet were van der Waals attractive forces. When the powder particles are much larger than 1 micron, their inertial forces are large compared to the van der Waals attractive forces and, hence, they exhibit little tendency to form agglomerates or pellets. As the particles become smaller, however, the ratio of attractive forces to inertial forces increases rapidly. As a consequence, the finer particles cling to each other at their points of contact to form agglomerates or pellets whose strength increases as the particle size decreases.
Meissner et al. stated that prior studies had also suggested that any dry powder will pelletize by mechanical agitation, such as that occurring by tumbling in a drum, if the prime particles are small enough. Moreover, the dry pelletization process is greatly facilitated by the presence of a sizable volume fraction of seed pellets. Any solid objects, nominally greater than 200-mesh (74 microns) in size, such as glass spheres, sugar crystals, metal shot, vegetable seeds, or recycled agglomerates of the powder itself, may serve as seed pellets. These grow at the expense of powder and prolonged tumbling ultimately causes all the loose powder to disappear. In other words, the seed pellets become coated with the submicron powder. Ford, L. H. and J. V. Shennan in "The Mechanism Of Binderless Granulation And Growth Of Ceramic Spheres," J. Nuclear Materials, 43, 143 (1972), showed that mixtures of submicron uranium oxide and carbon, having surface areas of 2 to 4 m.sup.2 /g and 17 to 30 m.sup.2 /g, respectively, can be dry pelletized. These workers used preformed pellets as seed pellets which grew at the expense of the unagglomerated powder.
Accordingly, the prior art has demonstrated that powders with sizes smaller than about one micron can be dry pelletized and that the pelletization process is greatly facilitated by the use of seed pellets. The seed pellets can consist of either the same material being pelletized or of a foreign material. With either class of seed pellets, the product pellets consist, predominantly of a seed core covered by a layer of the submicron powder.
Although the prior art has demonstrated that submicron powders can be dry pelletized, only carbon black appears to be pelletized on a large commercial scale by a dry tumbling process. Moreover, even with carbon black, use of foreign materials as seed pellets does not appear to be practiced on a commercial scale probably because they cannot be separated in an economically feasible manner from the carbon black in the product pellets. Further, incorporation of the foreign materials suggested as potential seed pellets, such as plant seeds, sugar crystals, metal shot and glass spheres, with the carbon black (or other submicron materials) in end use applications as, for example, in news inks, tire products and polymeric media would be detrimental to their performance properties.
Boysen et al. used a fluid bed process to form core-shell resin particles composed of a core containing a majority (more than 90% by weight) of rubber or resin and a shell containing a majority (more than 75% by weight) of particulate matter such as carbon black, silica, clay and other like materials. Further exemplification of this technology appears in Boysen, R. L., L. S. Scarola, and A. S. Rhee, "Core-Shell Resin Particle," U.S. Pat. No. 5,304,588 (1994). The rubber or resin core consisted of materials such as very low density polyethylene, ethylene/propylene diene monomer (EPDM), ethylene/propylene monomer (EPM) and polypropylene copolymers.
Wood introduced a countercurrent flow of rubber crumb in a stream of carbon black effluent gases containing carbon black to be recovered. This technology is further exemplified in Wood, J. Q., "Recovery Of Carbon Black," U.S. Pat. No. 2,719,135 (1955). Wood claimed that most of the carbon was removed from the gas stream by adhering to the surface of the rubber crumb. Moreover, the amount of carbon black adhering to the crumb could be controlled by recycling the crumb until it had acquired the coating desired. Thus the product formed by Wood can be characterized as consisting of a rubber core and a carbon shell. Further, the crumb-carbon black product could be used in a rubber compounding operation.
Pigments used to color thermoplastic polymeric media and/or provide flame retardancy frequently have submicron particle sizes. These include white pigments, such as titanium dioxide, zinc oxide, antimony oxide and hydrated aluminum oxide, black pigments, such as carbon blacks as well as colored organic and inorganic pigments, such as phthalocyanines, quinacradones, red iron oxides, cadmium sulfoselenides and chrome oxides or a suitable mixture thereof. In practice, the same equipment is often used to produce differently colored thermoplastic polymeric articles. In such applications, the pigments must be dispersed well so as to attain their full coloring values and to avoid degrading polymer properties. All of the aforementioned groups of submicron pigments are suitable for use in the present invention.
To avoid cross-contamination when using the same equipment with different powdered or pelletized pigments, extensive cleaning of the equipment is mandatory. Cleaning becomes particularly time consuming and costly when pigments which are prone to dustiness, such as carbon black, are used. To overcome the need for extensive cleaning, to ensure attainment of good quality dispersions, to attain accurate metering and to eliminate the possibility of dust formation, concentrated dispersions, termed masterbatches or concentrates, of the various pigments or pigment mixtures in the thermoplastic media are often used in place of either the powder or pelletized forms of the pigments.
The production of masterbatches using thermoplastic polymers, such as polyethylene, polypropylene, acrylonitrile-butadiene-styrene copolymer, ethylene vinyl acetate, etc., is of special importance. In this application, pellets of the thermoplastic polymer and pigment or pigments, either in powder form, such as in the case of titanium oxide, or pelletized form, such as in the case of carbon black, are fed to high shear mixers such as Banbury mixers, twin screw extruders or the like. As a result of the combination of heating and mechanical work, the polymer is transformed to a viscous molten state in which the pigment or pigments is dispersed. For production of acceptable masterbatches, the formation of good quality dispersions is of critical importance. After the dispersion process is complete, the masterbatch is, for example, extruded, cooled and then sliced into pellets for shipment.
Thermoplastic polymers manufactured by the "slurry process" are produced in a powder form, called "reactor crumb". Because of potential feeding problems, reactor crumb is not used as the preferred feed to shear mixers. Instead, prior to use in masterbatch formation, the crumb is, typically, converted to 3 mm cylindrical pellets (by means of a single or a twin screw extruder followed by slicing). Similarly, because of potential feeding problems and because of its low bulk density and in spite of its superior dispersibility, fluffy black is not normally employed in masterbatch production. Instead, spherically shaped carbon black pellets, formed either in pin or dry drum pelletizers, with mean sizes in the range of 0.2 to 2.0 mm are employed as feed. Thus the feeds employed in masterbatch formation consist of carbon black and polymer pellets having differing mean sizes. This disparity in size is even more pronounced with powdered pigments.
The disparity in the sizes (and shapes) of the thermoplastic polymer and pigment or pigments enhances segregation processes and, for this reason, formation of a uniform blend of the polymer and pigment feeds becomes difficult. Thus, for formation of good quality masterbatches both dispersive and distributive mixing are required.
Reactor crumb has both a size (mean sizes in the range of 0.2 to 2.0 mm) and a density of about 0.9 g/cc which makes it ideal for consideration as a seed material in the dry drum pelletization of submicron pigment powders. Further, the resultant co-pelletized pigment/polymer compositions, in the form of core and shell products, represent intimate mixtures of the polymer and pigment or pigments. In addition, the composition of the seed can be chosen so that it is identical with that used in making the masterbatch. Thus, with the appropriate choice of polymeric seed material, the co-pelletized pigment/polymer products are highly suited for directly feeding to mixers for forming pigment loaded masterbatches containing white pigments, such as titanium dioxide, zinc oxide, antimony oxide and hydrated aluminum oxide, black pigments, such as carbon blacks as well as colored organic and inorganic pigments, such as phthalocyanines, quinacradones, red iron oxides, cadmium sulfoselenides and chrome oxides or a suitable mixture thereof. When mixtures of pigments are utilized, any proportion of the submicron pigment powders may be used to form the co-pelletized mixed pigment/polymer product.
For economic reasons, high loadings of pigment in a concentrate are preferred. However, for rapid incorporation during let-down, the viscosity of the concentrate should not be very different from that of the medium in which it is being dispersed. Concentrate viscosity increases with pigment loading and approaches a high value as its solids content approaches that required for the pigment to attain its maximum packing fraction, defined as the true volume of the pigment divided by the apparent volume occupied by the pigment. Further details of the effects of loading on viscosity may be found in, for example, Nielsen, L. E., "Polymer Rheology," Marcell Dekker, Inc., New York, 1977. Accordingly, to obtain acceptable viscosities, the pigment loading in a masterbatch will be less than that at which it attains its maximum packing fraction.
A variety of methods are available for estimating the maximum packing fraction of pigments. A particularly convenient method is that based on the determination of their oil absorption values. Details for determining oil absorption values and procedures for estimating packing fractions from them are given in Patton, T. C., "Paint Flow And Pigment Dispersion," John Wiley & Sons, Inc., Second Ed. (1979). Since most pigmentary powders are composed of reasonably spherical unaggregated particles, they have relatively small oil absorption values, ranging from about 17 to about 50 cc oil per 100 g pigment and, hence, have maximum packing fractions which, typically, exceed 0.5. Masterbatches having pigment loadings well in excess of 50 weight % can be formed from such pigments without increasing their viscosities by more than a factor of 2 to 3 when compared to the unloaded polymer.
Carbon blacks, unlike most pigmentary particles, are composed of aggregates which, depending on the grade of black, can contain a large number of fused primary particles. The maximum packing fraction of carbon black can be estimated from its di-n-butyl phthalate, DBP, absorption which can be determined by the ASTM D 2414 procedure. The DBP and oil absorption tests provide similar information.
Experience has shown that carbon black pellet dispersibility decreases as black surface area increases and/or its DBP decreases. Because of difficulties encountered in their dispersion (and depending on the application), blacks with low DBP values and very high surface areas are rarely used to form masterbatches. For example, for applications where jetness or UV protection are needed, the black should have a high surface area. To form acceptable concentrates with a black having a high surface area, a high DBP product may often be used in concentrate formation. Thus, practical considerations dictate that in masterbatch formation a compromise be struck between black loading and dispersion quality. For this reason, blacks with the lowest attainable DBP values are rarely used in the production of black masterbatches.
In spite of their costs, the market for pigment concentrates or masterbatches is substantial because the resulting products are dust-free, easily conveyed, accurately metered and much more easily dispersed in compatible thermoplastic media than either pigment powders or conventionally pelletized blacks. We have found that the dry process pellets of the present invention, formed using reactor crumb as seed pellets, represent a very desirable raw material source to be used for the production of pigment loaded masterbatches. Further, in applications where the conveying and metering equipment can handle pellets with a wider size distribution than that of conventional masterbatch pellets, the products of the present invention can be used in place of masterbatch for introducing the desired submicron pigment or pigments into polymeric media without substantial loss in dispersion quality.